If you follow official documentation to translate your library you might end up without a way to localize your library.
I talk about official Apple’s website: https://developer.apple.com/documentation/xcode/localizing-package-resources
When you do everything as it’s listed there it won’t work.
Why?
Because one important thing is missing.
If your main application doesn’t set CFBundleAllowMixedLocalizations in .plist file to True (YES) then this whole localization won’t be applied.
The reason why this flag is important is very simple. Let’s say you support only English in your app and you want to use a third-party library which supports English, Spanish and French. Then if someone has the main language on the phone set to Spanish then content from you app will be in English but this third-party library will be in Spanish.